The invention relates generally to magnetic recording media and specifically to thermally stable, antiferromagnetically coupled exchange media.
Magnetic hard-disk drives (xe2x80x9cHDDsxe2x80x9d) can store and retrieve large amounts of information. The information is commonly stored as a series of bits on a stack of thin-film magnetic disk platters, each of which is an aluminum alloy or glass substrate coated on each side with a thin-film magnetic materials layers and one or more protective layers. A bit is identified as a flux transition. Linear bit density is measured as the number of flux transitions per unit length, and areal bit density is measured as the number of flux transitions per unit area. Typically, the higher the linear and areal bit densities, the lower the signal-to-noise ratio. Read-write heads, typically located on both sides of each platter, record and retrieve bits from circumferential tracks on the magnetic disks.
Although great strides have been made over the past decade in increasing the linear and areal bit densities of hard drives, information storage requirements have increased dramatically. An ongoing challenge of disk drive manufacturers is to provide even higher linear and areal bit densities and higher data recording rates for thin-film magnetic disks. To realize higher linear and areal bit densities and data recording rates, it is necessary to provide magnetic recording media having higher signal to noise ratios (SNR) and lower magnetization thickness products (xe2x80x9cMrtxe2x80x9d). As will be appreciated, the Mrt is the product of the remanent magnetization Mr, the magnetic moment per unit volume of ferromagnetic material, and the thickness t of the magnetic layer. These objectives have been realized by using smaller and smaller grain sizes in the magnetic layer. Average grain diameters are now less than 10 nm.
The use of smaller grain sizes had a detrimental impact on the thermal stability of grain magnetization, particularly at high bit densities where the demagnetizing fields are significant. The equation which determines the stability of a recording medium against thermal fluctuations is KuV/kBT, where Ku is the magnetic anisotropic energy of the magnetic medium, V is the volume of a magnetic grain, kB is Boltzmann""s constant, and T is the absolute temperature. Magnetic media having higher values for KuV/kBT are generally more stable against thermal fluctuations. When magnetic media have lower values and are therefore thermally unstable, increases in temperature can cause loss of stored information through the onset of the superparamagnetic effect. When a magnetic recording layer exhibits superparamagnetic behavior, the layer, in the remanent state (in the absence of an applied magnetic field), returns to its lowest energy state in which the magnetic domain states are randomly distributed. This random distribution typically causes the recording layer to have a zero or near zero average magnetic moment. Flux transitions recorded in the layer are generally lost when the layer behaves superparamagnetically.
Attempts to control thermal instability typically attempt to increase the value of the numerator in the above equation, namely KuV. In one approach, a higher anisotropy material is used to provide a higher value for Ku while maintaining the grain volume at a low level to realize desired linear and areal densities. However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too great to be written by a conventional recording head. As will be appreciated, the xe2x80x9ccoercivityxe2x80x9d of a magnetic material refers to the value of the magnetic field required to reduce the remanence magnetic flux to zero, i.e., the field required to erase a stored bit of information. In the other approach, the effective magnetic volume V of the magnetic grains is increased.
FIG. 1 shows a cross-section of a magnetic disk that provides an increased magnetic volume while maintaining a low Mrt. The disk employs a laminated information layer 100 formed above an underlayer 104 and supporting substrate 108. In the laminated information layer, the magnetic moments 112 and 116 in the upper and lower ferromagnetic films 120 and 124, respectively, are antiferromagnctically coupled together across a very thin (less than 10 xc3x85 thick) nonmagnetic spacer film 128 (which is typically pure (undoped) ruthenium). The anti-parallel orientations of the moments 112 and 116 add destructively to provide a low net magnetic moment for the laminated magnetic layer 100. The thermal stability of the laminated layer 100 is, theoretically, substantially enhanced because the grains in the lower magnetic layer 124 are magnetically coupled with the grains in the upper magnetic layer 120 and thus the physical volume of layers 120 and 124 add constructively to provide a higher value for V. Thus, the films can contain very small diameter grains while theoretically maintaining good thermal stability. However, the degree of the improvement in the thermal stability has been far less than expected, particularly when the boron content of the upper ferromagnetic layer (layer 120 in FIG. 1) exceeds 7 atomic %. Although buffer layers (i.e., various ferromagnetic Co-based layers) have been used between the spacer and ferromagnetic films 128 and 120 and 124 to provide enhanced thermal stability, the SNRs for such media have been substantially decreased, particularly when the buffer layer is inserted between the spacer film 128 and the upper ferromagnetic film 120. The decrease in SNR for such media is believed to be due to the high exchange coupling between grains within the ferromagnetic buffer layer. Compared to non-antiferromagnetically-coupled media, the laminated magnetic layer has a higher coercivity and lower writability due to the increased effective total magnetic layer thickness.
These and other needs are addressed by the various media embodiments and configurations of the present invention. The present invention is directed to a nonferromagnetic buffer film that is particularly useful in magnetic media having ferromagnetically or antiferromagnetically coupled ferromagnetic films.
In one medium configuration, the buffer film is paramagnetic and is located between a pair of ferromagnetic films. As will be appreciated, a paramagnetic material has a Curie temperature below room temperature (e.g., about 25xc2x0 C.) and displays similar magnetic behavior to a superparamagnetic material. A paramagnetic material typically has a relative permeability that is slightly greater than unity and independent of the magnetizing force.
In another medium configuration, the buffer film has superparamagnetic properties within the operating temperature range of the disk. To realize such properties, the buffer film can, for example, have a value of KuV that is no more than about 25 kT to provide the desired degree of thermal instability.
In yet another medium configuration, the buffer film is selected so as to provide epitaxial growth conditions for an adjacent and overlying ferromagnetic film. To realize such properties, the buffer film preferably has a lattice mismatch with the adjacent and overlying ferromagnetic film of no more than about 5%. The buffer film is nonferromagnetic. As used herein, a xe2x80x9cnonferromagneticxe2x80x9d material does not display ferromagnetic behavior under the operating temperature range of the disk. xe2x80x9cNonferromagneticxe2x80x9d materials may display paramagnetic or superparamagnetic behavior or be magnetically nonreactive.
The buffer films of the above configurations can be particularly useful in antiferromagnetically exchange coupled media, where thermal stability and noise have traditionally been problems. The use of a nonferromagnetic buffer film in AFC media can provide increased thermal stability (a higher KuV/kBT value) for the information layer compared to a AFC media which do not have the buffer film. This increase in thermal stability is more significant for boron contents in the ferromagnetic films in excess of 7 atomic %. The use of a paramagnetic or superparamagnetic buffer film can also increase the AFC medium""s remanence coercivity while reducing the instrinsic switching field compared to AFC media which do not have the buffer film. This can provide improved writability even at high recording rates. The use of a paramagnetic or superparamagnetic buffer film can maintain the AFC medium""s signal-to-noise ratio performance (unlike other methods (such as using ferromagnetic buffer films)) and the antiferromagnetic exchange coupling through a wide buffer film thickness range.
These and other advantages will be apparent from the disclosure of the invention(s) contained herein.
The above-described embodiments and configurations are neither complete nor exhaustive. As will be appreciated, other embodiments of the invention are possible utilizing, alone or in combination, one or more of the features set forth above or described in detail below.